Method of operating an electromagnetic flowmeter

Abstract

The flowmeter comprises a flow sensor with a flow tube, two coils, and two measuring electrodes as well as measuring and control electronics. The method serves to compensate interfering potentials that are caused by inductive and/or capacitive interference arising from the coil leads and/or by particles of foreign matter or air bubbles in the liquid to be measured and/or by coatings on the measuring electrodes. During first subcycles of the excitation current, a measurement signal proportional to volumetric flow rate is computed as usual. During second subcycles of the excitation current, at least one voltage pulse is applied to at least one of the measuring electrodes.

What is claimed is:  
     
       1. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of: 
       causing the fluid to flow through the flow tube;  
       causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow through a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid;  
       inducing a voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;  
       removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and  
       applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current through the coil assembly.  
     
     
       2. A method as claimed in  claim 1  wherein the discharge voltage is an at least intermittently periodic sequence of voltage pulses. 
     
     
       3. A method as claimed in  claim 2  wherein two successive voltage pulses are of like polarity. 
     
     
       4. A method as claimed in  claim 2  wherein two successive voltage pulses are of different polarity. 
     
     
       5. A method as claimed in  claim 2 , wherein the voltage pulses have a width substantially smaller than a pulse width of the excitation current. 
     
     
       6. A method as claimed in  claim 2 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       7. A method as claimed in  claim 3 , wherein the voltage pulses have a width substantially smaller than a pulse width of the excitation current. 
     
     
       8. A method as claimed in  claim 3 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       9. A method as claimed in  claim 4 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       10. A method as claimed in  claim 1 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       11. An electromagnetic flowmeter for a fluid flowing in a line, comprising: 
       a flow tube connectable into the line for conducting the fluid;  
       a measuring and control circuit;  
       means fed by said measuring and control circuit for producing a magnetic field cutting across said flow tube with a coil assembly mounted on said low tube and traversed by an excitation current, said excitation current being clocked periodically;  
       at least two measuring electrodes for picking off potentials which are induced in the fluid flowing through said flow tube, which fluid is penetrated by the magnetic field;  
       means connected at least intermittently to said measuring electrodes for generating at least one measurement signal derived from the potentials induced in the fluid; and  
       means controlled by said measuring and control circuit for generating a discharge voltage at the measuring electrodes, said discharge voltage being a periodic sequence of voltage pulses, and said means for generating a discharge voltage in the form of voltage pulses being controlled such that the voltage pulses are applied to said measuring electrodes when the excitation current is different from zero, wherein:  
       a period of said clocked excitation current is different from a period of said sequence of voltage pulses.  
     
     
       12. The flowmeter as claimed in  claim 11 , wherein the period of the clocked excitation current is greater than the period of the sequence of voltage pulses. 
     
     
       13. The flowmeter as claimed in  claim 11 , wherein two successive voltage pulses are of like polarity. 
     
     
       14. The flowmeter as claimed in  claim 11 , wherein two succesive voltage pulses are of different polarity. 
     
     
       15. The flowmeter as claimed in  claim 11 , wherein the voltage pulses have a pulse width being substantially smaller than a pulse width of the excitation current. 
     
     
       16. the flowmeter as claimed in  claim 11 , wherein the voltage oulses are applied intermittently to both measuring electrodes. 
     
     
       17. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of: 
       causing the fluid to flow through the flow tube;  
       causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow though a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid;  
       inducing a voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;  
       removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and  
       applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current though the coil assembly, wherein:  
       the discharge voltage is an at least intermittently periodic sequence of voltage pulses; and  
       the excitation current is at least intermittently clocked periodically with a predetermined period, with the period of the clocked excitation current being different from a period of the sequence of voltage pulses.  
     
     
       18. A method as claimed in  claim 17 , wherein the period of the clocked excitation current is greater than the period of the sequence of voltage pulses. 
     
     
       19. A method as claimed in  claim 18 , wherein the voltage pulses have a width substantially smaller than a pulse width of the excitation current. 
     
     
       20. A method as claimed in  claim 18 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       21. A method as claimed in  claim 17 , wherein the voltage pulses have a width substantially smaller than a pulse width of the excitation current. 
     
     
       22. A method as claimed in  claim 17 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       23. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of causing the fluid to flow though the flow tube; 
       causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow though a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid;  
       inducing a voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;  
       removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and  
       applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current through the coil assembly, wherein:  
       The voltage pulses have a width substantially smaller than a pulse width of the excitation current.  
     
     
       24. A method as claimed in  claim 23 , wherein voltage pulses are applied intermittently to both measuring electrodes. 
     
     
       25. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of: 
       causing the fluid to flow through the flow tube;  
       causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow through a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid;  
       inducing a voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;  
       removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and  
       applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current through the coil assembly, wherein:  
       the discharge voltage is an at least intermittently periodic sequence of voltage pulses;  
       two successive voltage pulses are of like polarity; and  
       The voltage pulses have a width substantially smaller than a pulse width of the excitation current.  
     
     
       26. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of: 
       causing the fluid to flow through the flow tube;  
       causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow through a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid′ 
       inducing voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;  
       removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and  
       applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current through the coil assembly, wherein:  
       the discharge voltage is an at least intermittently periodic sequence of voltage pulses;  
       two successive voltage pulses are of different polarity; and  
       voltage pulses are applied intermittently to both measuring electrodes.